1. Field of the Invention
This invention relates to a process for the production of magnetic recording elements, more particularly to a process for producing magnetic recording elements such as magnetic tapes, sheets, discs and cards, etc., in which a finely powdered ferromagnetic metal(s) is/are used for magnetic recording, e.g., sound-recording, image-recording, memory, etc.
2. Description of the Prior Art
Ferromagnetic powders hitherto in magnetic recording elements include .gamma.-Fe.sub.2 O.sub.3, Co-containing .gamma.-FeO.sub.3, Fe.sub.3 O.sub.4, Co-containing Fe.sub.3 O.sub.4, Bertholide iron oxide, Co-containing Bertholide iron oxide, CrO.sub.2 and the like. These ferromagnetic substances, however, suffer from limitations in recording magnetic signals having a short recording wave length or narrowed track width, that is, when used in high density recording they are not satisfactory in magnetic characteristics such as coercive force (Hc: Oe) and maximum residual magnetic reflux density (Br: Gauss).
Ferromagnetic substances having suitable characteristics for the high density recording include finely powdered ferromagnetic metals. Magnetic tapes produced using such ferromagnetic metal fine powder are in practical use as "metal tape" and have received increasing attention in the field of audio cassettes.
The following methods of producing such ferromagnetic metal fine powders are known:
(1) Organic acid salts of ferromagnetic metals are heat-decomposed and reduced in reducing gases, as described, for example, in U.S. Pat. Nos. 3,186,829 and 3,190,748. PA1 (2) Needle-shaped oxyhydroxides, mixtures of such needle-shaped oxyhydroxides and other metals, or needle-shaped iron oxides obtained from such oxyhydroxides are reduced, as described, for example, in U.S. Pat. Nos. 3,598,568, 3,634,063, 3,607,219, 3,607,220 and 3,702,270. PA1 (3) Ferromagnetic metals are evaporated in low-pressure inert gases, as described, for example, in Japanese Patent Applications (OPI) 25662/73 and 25663/73. PA1 (4) Metal carbonyl compounds are decomposed by heating, as described, for example, in U.S. Pat. Nos. 2,983,997, 3,172,776, 3,200,007 and 3,228,882. PA1 (5) Fine ferromagnetic metal powders are electrically precipitated using a mercury cathode and then separated from mercury, as described, for example, in U.S. Pat. Nos. 3,198,777, 3,156,650 and 3,262,812. PA1 (6) Salts of metals capable of producing ferromagnetic substances are reduced in aqueous solution by reducing substances such as, for example, borohydride compounds, hypophosphites, hydrazine, etc., as described, for example, in U.S. Pat. Nos. 3,607,218, 3,756,866, 3,206,338, 3,494,760, 3,535,104, 3,567,525, 3,661,556, 3,663,318, 3,669,643, 3,672,867, 3,700,499, 3,726,664, 3,837,912, 3,865,627, 3,932,293, 3,943,012, 3,966,510, 4,007,072, 4,009,111, 4,020,236 and 4,074,012. PA1 (1) Areas of a magnetic recording element formed therefrom which are in contact with a magnetic head, a cylinder, a tape guide, etc., at high temperature and humidity are subject to corrosion or discoloration. PA1 (2) The surface of the magnetic layer of the magnetic recording element deteriorates with time. PA1 (3) When the tape is dipped in brine, rust is easily formed. PA1 (1) Corrosion or discoloration of those parts which are in contact with a magnetic tape, such as a magnetic head, a cylindrical drum, a guide pole, etc. of magnetic recording devices (e.g., an audio tape recorder and a video tape recorder) is prevented. PA1 (2) Deterioration of the surface of a maganetic layer is reduced and the practical quality is increased. PA1 (3) The formation of rust is prevented. PA1 (4) Stability of the ferromagnetic metal fine powder against oxidation and safety during transportation and storage are increased. PA1 (5) Because of the good compatibility of the ferromagnetic metal fine powder with binders, a wide variety of binders can be utilized and a ferromagnetic metal fine powder having excellent dispersibility is obtained. PA1 (6) A magnetic recording element having excellent electromagnetic conversion characteristics is obtained.
Methods (1) to (4) are grouped as dry methods while methods (5) and (6) are grouped as wet methods.
While the above described ferromagnetic metal fine powders (or ferromagnetic alloy powders) have excellent magnetic characteristics in comparison with the above described ferromagnetic fine powders of the oxide type (for example, .gamma.-Fe.sub.2 O.sub.3, Fe.sub.3 O.sub.4, CrO.sub.2 and these oxides with other metals added thereto), they are easily subject to oxidation, are chemically unstable and deteriorate in magnetic characteristics with the passage of time. Furthermore, since they may ignite when allowed to stand in air in the state of a powder, they are specified as a dangerous article in Group 2 of the Japanese Fire Defence Law.
To increase the recording density of magnetic recording elements, it is desirable to minimize magnetic particle size; however, when particle size is made smaller, the above described problems occur more markedly.
Therefore, there has been employed a method where, after formation of a ferromagnetic metal fine powder, a protective layer is provided on the surface of the metal particles by surface-treatment to increase chemical stability. For example, the following are known: treating with oxidizing substances such as chromic acid salts, permanganic acid salts, etc. (Japanese Patent Applications (OPI) 112465/76 and 5038/78); coating with an organic substance (Japanese Patent Applications (OPI) 21251/77, 155398/77, 141202/78 and 76958/78), and treating with a reactive gas (Japanese Patent Applications (OPI) 123601/74 and 85054/77).
Particularly, in the case of ferromagnetic metal fine powders produced by dry methods (1) to (4), the above stabilization treatment is of importance. In dry methods it is known that metals easily catch fire as a result of abrupt oxidation after production thereof as fresh surfaces of the metals are directly exposed to air.
To stabilize metal particles produced by dry methods there is generally employed a method where by gradually increasing the oxygen pressure in the atmosphere after formation of the particles an oxide layer is provided on the surface of the particles, thus preventing metals from igniting. In addition to oxygen, reactive gases such as H.sub.2 S can also be used.
It has further been confirmed that insufficient stabilization results in the problem that even after formation of a magnetic layer (metals in combination with a binder) the metals easily catch fire at relatively low temperatures, that the formation of a uniform oxide layer generally improves wetting between such a binder and the oxide layer (permitting the formation of a magnetic layer having good planar properties) and furthermore that deterioration in magnetic characteristics of a metal powder and metal powderbased recording elements proceeds easilywith the passage of time.
For safety purposes, ferromagnetic metal fine powders subjected to the above stabilization treatments are sometimes dipped in a solvent and then handled.
It is also known that magnetic recording elements produced using magnetic powders containing SO.sub.4.sup.-- and Cl.sup.- are adversely affected in magnetic recording capability by the SO.sub.4.sup.-- and Cl.sup.-. To eliminate such a problem, So.sub.4.sup.-- and Cl.sup.- removal procedures have been proposed (see Japanese Patent Publications 11733/75 and 27118/73).
Additionally, Japanese Patent Publication 11733/75 discloses a method in which goethite or .gamma.-Fe.sub.2 O.sub.3 is heated in air at 600.degree.-800.degree. C. for more than 1 hour to dissipate SO.sub.4.sup.-- as SO.sub.3 or SO.sub.2 gas, and Japanese Patent Publication 27118/73 discloses washing with an aqueous medium to remove water-soluble impurities after conversion into magnetite or maghemite.
It has now been found that even after SO.sub.4.sup.-- and Cl.sup.- have been removed by the above described methds, at steps where magnetic powders are reduced to ferromagnetic metal fine powders, SO.sub.4.sup.--, Cl.sup.-, K.sup.+ and Na.sup.+ result as impurities and enter into the ferromagnetic metal fine powders formed, i.e., even though these impurities are removed while processing in the state of iron oxyhydroxide, or magnetite or maghemite, such impurities which exist in the interior of the crystals of the starting materials are released when the starting materials are reduced to the metal state. Further, this tendency is marked when starting materials modified by Co, Ni, etc., rather than pure Fe particles are used, and is increased when the proportions of Co, Ni, etc., are high.
Furthermore, it has been found that when reduction is continued until the ferromagnetic metal fine powders are obtained, water-soluble salts of metals comprising the ferromagnetic metal (for example, iron, cobalt, nickel) and other added metals (for example, chromium, manganese, zinc, etc.) are formed.
In general, ferromagnetic metal fine powders have hitherto often been dipped in organic solvents prior to handling; however, water-soluble components containing SO.sub.4.sup.--, Cl.sup.-, Na.sup.+, K.sup.+, etc., are not removed and remain as impurities in the ferromagnetic metal fine powders to yield the following disadvantages: